The present invention relates to a reference electrode for electrochemical measurement or determination of the oxygen partial pressure in an ionic melt, especially in oxidic glass melts.
Observation of the quality of glass melts by electrochemical determination of oxygen partial pressure is known. Thus the oxygen partial pressure provides information regarding the redox state of the glass melt. All the properties of the glass melt are effected by the redox state, which can be adjusted or set by the redox system, for example the purity or the color of the melt, in order to name only a few examples. Further details regarding this are provided in, among other references, F. G. K. Baucke, "High-Temperature Sensors for Oxidic Glass-Forming Melts" in Sensors--A Comprehensive Survey; Vol. 3; Chemical and Biochemical Sensors Part II; 1992, pp. 1156-80.
So-called ZrO.sub.2 -reference electrodes made from yittrium-stabilized Zirconium dioxide have been used up to now for measurement of the oxygen partial pressure, pO.sub.2, in glass melts. These known electrodes, on which the claimed invention is based, have been described many times in the technical literature, for example in the above-named prior art reference of Baucke.
The electrochemical cell including its ingredients used to measure the oxygen partial pressure may be represented by EQU Pt, O.sub.2 (1)/ZrO.sub.2 +xY.sub.2 O.sub.3 /Melt, O.sub.2 (2)/Pt.
The EMF(electromotive force), E, of this electrochemical cell can be used to obtain the partial pressure of oxygen, pO.sub.2, in the melt according to the formula (1) below: EQU pO.sub.2 (2)=pO.sub.2 (1).multidot.exp(-4EF/RT) (1)
where F=Faraday's Constant, R=the gas constant and T=the temperature.
It is essential for the operation of ZrO.sub.2 -reference electrode that a defined oxygen partial pressure, pO.sub.2 (1), exists at the three-phase boundary Pt(1),O.sub.2 (1),ZrO.sub.2 +xY.sub.2 O.sub.3. This is most easily guaranteed by a defined gas flow (predominantly air or pure oxygen) or by suitable metal/metal oxide buffer.
A half pipe or a massive cylindrical pin made from Zirconium oxide, which is held by a passage in a cylindrical aluminum oxide pipe, is usually used as the ZrO.sub.2 -reference electrode. The platinum electrode surrounded by the above-mentioned metal/metal oxide buffer or the oxygen and/or air surrounding atmosphere is located in the interior of the zirconium dioxide pipe and/or ends above the cylindrical massive pin in the aluminum oxide pipe so that contact between the platinum electrode and the zirconium dioxide pipe and/or pin is usually improved by embedding the platinum electrode in a zirconium dioxide powder.
An ideal measurement runs so that exactly 1 mol of oxygen is reacted by "infinitely slow passage" of 4F electrons through the measuring instrument and expands reversibly from higher to lower pressure.
The ceramic Zirconium dioxide in the measurement probe has both a separating and a conducting function. One the one hand it prevents the direct exchange of neutral oxygen and electrons (short circuiting of the cell), and on the other hand it allows the above-described transport of ions (electrolytic key) as an almost pure oxygen ion current. Furthermore Zirconium oxide is in general genuinely stable in regard to corrosion of the glass melt.
These advantages of zirconium oxide have been known for a long time and lead to even more wide usage of it in thermodynamic investigations of solid oxides. The high temperature change stability and corrosion resistance requirements in oxidic glass melts have long prevented their routine use in laboratory glass, especially in fusion by glass furnaces. Problem-free laboratory measurements were possible due to commercial distribution of the Yttrium-stabilized half pipes and rods. Use in glass melt-containing vessels, in which the conditions of usage necessarily require special structure and precautions, has not proven to be so easy. Meantime also a standard solution for this problem does exist (Muller-Simon et.al., Glastech. Ber. 64, 49-51 (1991)).
All available structural forms change nothing regarding the basic weaknesses of the Zirconium dioxide probe: it has a high sensitivity to mechanical and especially thermal stresses. Thus it is difficult to find a suitable mounting location, measurement in melt containers or in pure regions is not practical; and positive results occur only for feed ducts.
A break of the ceramic materials can occur particularly during introduction to a hot melt-containing vessel and/or in retempering or retempering of previously cooled vessels. A breakage or fracture makes the probe ineffective. Broken pieces of ceramics can lead by stone build-up to mechanical blockages in stirring vessels and feed units and to disadvantageous manufacturing stoppages.
Furthermore in a few glass melts the corrosion of the Zirconium oxide is so strong that a reproducible measurement of the pO.sub.2 is not possible.